A variety of alarm apparatuses are known which use electromagnetic or ultrasonic waves in order to irradiate a certain location and to measure the reflections received from said irradiation. Be it ultrasonic devices or infrared devices, all of them have one feature in common: they only work within the limits of the walls of a room or completely outside. As the type of radiation commonly used in such known alarm devices does not traverse the walls of conventional buildings, the surveillance of the surroundings of a room can be implemented only by radiation sources located outside the room and only by a plurality of such sources which cover all angles and spots of the surroundings similar to the installation of a plurality of video cameras which need to be positioned according to the same principle if one wishes to obtain absolute protection of the room.
It is evident, that this limitation of conventional alarm systems either lead to incomplete surveillance or to high costs produced by the number of devices to be installed in order to obtain complete surveillance.
Further, known alarm systems trigger alarm signals as soon as the reflection pattern in the room or the surveyed space changes and are therefore not usefull for applications where certain changes of this pattern should be permitted without producing an alarm such as the protection of a room in which a conference is held and wherein people may move, however where the approaching of a person outside the walls of said room should be indicated in order to alert the participants of the conference that somebody may listen.